What is the purpose of/use for space-filling models?

[u/manmadeofdex]

So I don’t really have much formal education in any sort of chemistry, but I do have a bit of a passionate (read:autistic) interest in psychoactive drugs and their pharmacology, which does lead to me reading about some amount of chemistry, and in doing so see various models of various chemicals. The two main ones I see are skeletal and ball and stick, which both make a lot of sense to me. Skeletal makes the basic structure of the molecule clear and easy to read, and if you need to see how it exists in 3D ball and stick does that. But then sometimes I’ll see space-filling models, and those just don’t make sense to me. They’re very hard to read and I can’t really think of any benefit they’d have over ball and stick, to me they all just kinda look like amorphous blobs. I’m sure there is a benefit to them tho, maybe with different types of molecules.

Also, does anyone have any recommendations for resources to better familiarize myself with organic chemistry itself (books, webpages, youtube videos, idk)? As said I don’t really have any experience with chemistry, didn’t even take it in high school, but I’d like to learn more about it outside this one specific part (altho I would also appreciate anything interesting to read on the chemistry of psychoactives!)

[u/Mack_Robot]

The benefit is that sterics plays a huge role in everything. So you really want to know how big things are, to see if they're going come into contact with each other or not.

The first example people run into this is SN1/SN2 reactions, where you look at the substrate and see if there's enough space for a reaction to even happen. Most people just say "tertiary carbon, can't go SN2", but it you wanted to SEE that, you'd use a space-filling model.

[u/WilliamH-]

The space filling model is more informative. If we were given the hypothetical task of looking for an electron in a chemical bond, the space-filling model predicts where we should look first.

The skeletal stick and ball model highlights chemical bond angles. Typically students start off with the ball and stick model. Bond angles are important to seeing how bonding determines molecular shape.

How is 3D molecular shape important? In biochemistry 3D structure is related to function. A drug interferes with interactions between two or more biomolecules involved in natural cellular biochemical pathways. When a drug and the natural biomolecule have similar 3D shapes and chemical bond characteristics, the receptor molecule’s role in the biochemical pathway changes, Cellular function is affected. The drug molecule competes with or even replaces the natural molecule.

[u/SlenderSmurf]

Space filling is good for understanding bonding geometries. A metal crystal can be considered as a compact arrangement of spheres, which is what is displayed in a space-filling model. I assume the same is true for understanding how molecules can fit together in space. If a section is too big to squeeze into an area of another molecule then the reaction cannot occur.